I'm pretty sure I've posted on this topic before.

My ICBM created a huge hum problem. It's the wallwart power supply. Like most, it's cheap - and noisy.

The solution to all my wallwart problems, was to buy an isolating transformer (otherwise known as a groundloop isolating transformer, or a 1:1 A/C transformer) and to run all of my wallwarts off a power bar connected to it. The one I have is pretty beefy but limited to 300 Watts. But that's not a problem because most wallwarts draw very little current. The only other thing to note is that these transformers typically run hot. It becomes a 20-30 pound hot block of iron and copper in no time. Keep it physically away from other stuff. Use it as a baseboard heater in winter as well. (Only mildly kidding.)

Jeff Mackwood